The present invention relates to an apparatus for applying a release agent onto the surface of a fixing roll of, for example, a plain paper copying (PPC) machine.
In a fixing mechanism of a PPC machine, paper bearing a transferred toner image is passed between a heated metal roll and a rubber roll or other elastic rolls under pressure at a temperature between 150.degree. and 200.degree. C. until the image is fixed. In this mechanism, it frequently occurs that the paper bearing a toner image is wound around the metal roll or elastic roll thereby preventing complete printing or jamming the machine. To avoid these problems, various techniques have been proposed, among which is the use of a fluorocarbon resin coating on the surface of the metal roll.
Whether a fluorocarbon resin coating is applied or not, all conventional types of fixing mechanisms apply an additional coating of a release agent onto the surfaces of the metal roll and elastic roll. However, it is very difficult to apply a release agent coating uniformly and in the right amount. If the amount is too large, it forms a blotch on the paper or discolors the paper. At the same time, rapid consumption of the release agent requires frequent and uneconomical refilling. Furthermore, excess release agent coagulated on the surface of the roll can cause unexpected trouble with the machine. If too small an amount of the release agent is applied, the releasability of the roll surface from paper is unavoidably reduced leading to the problem of the paper being wound around the roll. Typical release agents are silicone oil and other heat-resistant oils.
U.S. Pat. Nos. 3,718,116 and 3,745,972 disclose an applicator for the release agent which uses a two-layered felt, a dense felt layer and a loose felt layer. Use of this applicator prevents excessive application of the release agent by contacting a roll and the surface of the dense felt layer. However, the amount of release oil applied using this applicator is still large, particularly when its viscosity is low. A more serious defect with this applicator is that the felt tends to become clogged by toner or carrier particles which causes various problems in fixing due, for example, to scratches on the surface of the fixing roll or uneven application of the release oil. As a consequence, the felt must be replaced frequently which is not only uneconomical but also makes consistent operation of the fixing apparatus difficult.
A fixing apparatus that is designed to eliminate this defect is described in Japanese Published Patent Application No. 110049/77. The characteristic feature of this apparatus that the dense felt layer is eliminated and instead at least part of the felt structure is replaced by tetrafluoroethylene resin foam. This arrangement is inteneded to prevent excessive application of the release oil. But, since the pore size of the tetrafluoroethylene resin foam is not controlled satisfactorily in this apparatus, the amount of release oil applied is still great. Furthermore, the use of a large heat-resistant felt structure makes the overall apparatus complex, large in size, and costly. Moreover, because the felt absorbs the oil, not only does it take a long time before the application of the oil onto the fixing roll can be started, but also the oil absorbed by and held in the felt is entirely wasted. A more serious defect is that the method of fixing the tetrafluoroethylene resin foam involves problems to be solved.